Process for the production of aromatic dicarboxylic and polycarboxylic acids



3,042,717 PRQCESS F011 TEE ?RQDUCTION F ARO- IWA'IIC DICARBOXYLIC AND PGLYCARBOX- YLIC ACIDS Walter Schenk, Heidelberg, Germany, assignor to Henkel & (lie G.m.b.H., Dusseldorf-Holthausen, Germany, a corporation of Germany No Drawing. Filed Feb. 15, 1961, Ser. No. 89,344 12 Claims. (Cl. 260522) This invention relates to a process for the production of aromatic dicarboxylic and polycarboxylic acids, and more particularly to the production of aromatic dicarboxylic and polycarboxylic acids from alkali metal salts of other aromatic carboxylic acids having at least three carboxyl groups in the molecule.

I have found that industrially useful aromatic dicarboxylic and polycarboxylic acids are obtained in a very simple fashion by heating alkali metal salts of aromatic carboxylic acids having at least three carboxyl groups in the molecule to temperatures above 300 C. up to about 500 C., and thereafter converting the alkali metal smts obtained thereby into the corresponding free acids or various salts of such acids.

Aromatic polycarboxylic acids which may be employed as starting materials for the process according to the present invention are, for example, hemi-mellitic acid, trimellitic acid, trimesitinic acid, mellophanic acid, prehnitic acid pyromellitic acid, benzene-pentacarboxylic acid and mellitic acid, as well as mixtures of such acids. Mixtures containing these acids are produced, for instance, by oxidation of alkylbenzenes or by oxidation-degradation of higher, possibly alkylated, ring systems, or from carbonaceous substances such as graphite, mineral coal, brown coal, peat, wood, lignin, coal extracts, tars, pitch, asphalts, coke, petroleum residues or their transformation products by treatment with nitric acid or oxygen and alkalis. When benzene polycarboxylic acids are used, the product is predominantly terephthalic acid.

Further suitable as starting materials for the process according to the present invention are those aromatic polycarboxylic acids which are derived from polycyclic ring systems, especially bicyclic aromatic ring systems, such as diphenyl-2,3,4-tricarboxylic acid, naphthalenel,4,5-tricarboxylic acid, diphenyl-Z,3,5,6-tetracarboxylic acid, diphenyl-3,4,3,4'-tetracarboxylic acid, naphthalenel,4,5,8-tetracarboxylic acid and the like.

The above indicated aromatic carboxylic acids or the mixtures thereof are employed as starting materials in the form of their alkali metal salts. These salts may be produced by well known methods, such as by neutralization of the particular aromatic carboxylic acid with an alkali metal hydroxide to form the salt which is to be later used as the starting material. The potassium and sodium salts are especially suitable, because they are readily accessible and relatively inexpensive to produce. While the lithium, rubidium and cesium salts, as well as thallium salts, produce equally satisfactory results, their use as starting materials for the process herein disclosed is of only secondary importance because of their relatively high cost. In some cases itis advantageous to employ the acid alkali metal salts of the aromatic polycarboxylic acids as the starting material in place of the neutral alkali metal salts. 7

It is not necessary that the starting material contain the finished alkali metal salts of aromatic carboxylic acids. Equally suitable are reaction mixtures which, under the conditions of the reaction, produce the alkali metal salts in situ. For example, I have, found that the desired results are also produced by heating mixtures of aromatic carboxylic anhydrides or esters and suittes Patent able alkali metal salts, especially alkali metal carbonates, under the conditions above set forth. Such mixtures need not contain the alkali metal salt'forming components in exact stoichiometric proportions; one or the other metal salts, such as alkali metal carbonates, it is not' possible to conduct the reaction in the presence of alkali metal hydroxides. Thus, such strong alkalis cannot be in the reaction chamber during the reaction.

The presence of strong alkali in the reaction chamber produces an aromatic monocarboxylic salt. I have found that the reaction may be controlled by the precise conditions of my process to prevent splitting off of all but one carboxyl group so that an aromatic dicarboxylic acid salt is obtained.

The best results are obtained if the starting material is provided in a thoroughly dry state. If the alkali metal salts of the aromatic carboxylic acids serving as the starting materials are produced, for example, by neutralizing aqueous solutions of the carboxylic acids with alkali metal hydroxide, the dissolved alkali metal salts formed thereby may be transformed into dry powders by any suitable drying process, for instance by spray drying, drum-drying and the like. If necessary, the substantially dry powder may then be subjected to a further drying procedure just prior to its use in the present process, especially if the powder has been stored, in order to remove small residual amounts of moisture.

I have further found that the reaction according to the present invention is favorably influenced by the presence of catalysts. Metals and their compounds in general have proved to be suitable catalysts, especially zinc, cadmium, mercury, iron, lead, manganese and cesium and their compounds, such as their oxides, inorganic or organic acid salts, complexes and metal organic compounds. The amount of catalyst added to the starting material to produce the desired catalytic efiect may vary within rather wide limits, namely from 0 to 15% by weight, but preferably from 0.5 to 5% by weight, based on the Weight of starting material. Most advantageously, the catalyst is provided in a finely divided state and uniformly distributed throughout the starting material, which may, for example, be accomplished by dissolving or suspending the catalyst in an aqueous solution of the salts serving as the starting material and thereafter spraydrying, drum-drying or otherwise evaporating the suspension or solution to produce a dry, finely divided, homogeneous powder. However, the catalyst may also be added to the starting material in conjunction with well known carrier substances, such as kieselguhr.

In addition to catalysts, the reaction mixture may also The rearrangement reaction according to the present invention takes place upon heating the starting material to temperaturesabove 300 C. up to the temperaturev at In place of the inert solids, inert liquids may also Such suitable inert liquids are, for example, diphenyloxide, diphenyl, benzene,

naphthalene and the like.

which the salts of the aromatic carboxylic acids and the reaction products begin to decompose, but below the temperature of substantial decomposition most advantageously by heating the starting material to between 340 C. and 450 C. At temperatures above 500 C. the starting material as well as the reaction products decompose to' an excessive extent, so that the yields are substantially reduced. Consequently, it is not advantageous to carry out the reaction at such extremely elevated temperatures.

In order to avoid local overheating and sinterin g of the reaction mixture, it is advantageous to agitate the starting materials, for example by heating the reaction mixture in' autoclaves provided with a stirring device, in rotary autoclaves, in rotary furnaces or in fluidized bed systems. Similarly, adequate uniform heat distribution may be provided by distributing the reaction mixture in thin layers,

either in conjunction with or without agitation. However,

good yields are also obtained without the application of any of these measures, as long as means are provided to prevent local overheating.

The best results are obtained if oxygen is substantially V excluded from the reaction space during the rearrangement reaction according to the present invention. For this purpose it is advantageous to heat the starting material in the presence of inert gases such as carbon dioxide, nitrogen, methane,-benzene, carbon monoxide and the like. Particularly good yields are obtained if the rearrangement reaction above described is carried out in an atmosphere of carbon dioxide under pressure. However, elevated pressures are not essential to satisfactory yields; the rearrangement reaction will also proceed at subatrnospheric and atmospheric pressures.

. Thereafter, the dry product was treated for an hour and a half in a fluidized bed at a temperature of 390 C. and a carbon dioxide pressure of 80 atmospheres. The carbon dioxide was recycled with the aid of a recycling pump. The reaction product was then worked up as described in Example I and yielded 65.5 parts terephthalic acid.

Example III 15 parts of the tripotassium salt of hemimellitic acid,

admixed with 0.75 part cadmium oxide, were heated at 470 C. for five minutes in a glass vessel on an aluminum block (the temperature was measured in the aluminum block). During the run, carbon dioxide was passed over the reaction mixture. The raw product thus obtained was extracted with boiling water and after filtration the resulting solution was admixed with hydrochloric acid. .The precipitated terephthalic acid was repeatedly washed with hot Water and subsequently dried at 130 C. The

. yield was 4.2 parts of terephthalic acid.

The various dicarboxylic and polycarboxylic acid alkali metal salts formed by the rearrangement reaction may be separated from each other, from untransformed starting material and from the catalyst by a number of known methods. prises dissolving the reaction product mixture in water, filtering ofI' insoluble components, precipitating the acids or their acid alkali metal salts by acidifying the filtrate with acid agents such as sulfuric acid, hydrochloric acid or carbon dioxide, and separating the precipitated acids or acid salts from each other, for example by extraction with hot water. Any untransformed starting materials may readily be recovered from the aqueous solution and may be reused as starting materials for subsequent rearrangement reactions, The free acids or their alkali metalsalts may, if desired, be-transformed into their derivatives such as their methyl esters by methods well known in chemical industry.

The following examples will further illustrate my invention and enable persons skilled in the art to understand the invention more completely; It is understood, however, that the invention is not limited to these particular xamp es.

e Example I 165 parts of the dipotassium salt'of pyromellitic'acid and 5 parts potassium carbonate were heated for one hour at about 150 C. in a rotary autoclave in order to remove traces of moisture. Thereafter, carbon dioxide was introduced to a pressure of 50' atmospheres gauge and the autoclave was heated' to about 400 C. for six hours. During this time the pressure rose 'to about 122 atmospheres. After cooling, the reaction mixture was dissolved in 100 parts water and the terephthalic acid was precipitated-from the solution by' acidification thereof with suluric acid. The precipitated acid wasseparated from the hot solution, which contained the untransformed carboxylic acid mixture. 61.5 parts terephthalic acid were ob tained from'this' first run, and by subjecting the benzenepolycarboxylic acid mixture to-the above reaction a second 'time,17.9 additional parts of; terephthalic acid were obtainedin such pure form thatit could'be used for the production of polyesters without further purification.

For example, a very suitable method com- Example IV 15 parts of the tripotassium salt of trimellitic acid, admixed with 0.75 part cadium carbonate, were heated for five minutes at 420 C., as described in Example III. The raw product was then worked up in the manner described in that example and yielded 4.2 parts of terephthalic acid.

Example V v40 parts of the tripotassium salt of hemimellitic acid in admixture with 4 parts cadmium benzoate were heated for one hour at 420 C. in a rotary autoclave in an atmosphere of carbon dioxide under pressure. The initial pressure of carbon dioxide was 58 atmospheres and the final pressure was 194 atmospheres. Upon'working up the reaction product in the above described manner, 12.5 parts terephthalic acid were obtained.

Example Vl 10.0 gm. trimesitinic acid, together with 30.0 gm. potas-. sium carbonate and 2.0 gm. cadmium fluoride, were milled and the resulting mixture was heated in an autoclave at an initial carbon dioxide pressure of 50 atmospheres for four hours at 420 C. Upon working up the reaction mixture in the above described manner, 6.3 gm.

terephthalic acid were obtained. r

Example VII described manner and yielded 2.7. gm. terephthalic acid.

a Example VIII An intimate mixture of 25.0 gm. of the tripotassium salt of trimesitinic acid with 25.0 gm. potassium carbonate and 4.0 gm. zinc fluoride was heated for three hours at 340 C. at an initialcarbon dioxide pressure of 50 atmospheres. The reaction mixture was worked up in the above described manner and yielded 7.3 gm. terephthalic acid.

I Example IX A mixture of 25.0 gm. of the potassium 'salt of trimesitinic acid with 25.0, gm. potassium carbonate and 2.0 gm. cadmium fluoride, was heated for three hours at 430 C. at an'initial nitrogen pressure of atmospheres.

The reaction mixture was worked up in the above described manner and yielded 9.0 gm. terephthalic acid.

While I have disclosed certain specific embodiments of my invention, it will be apparent to persons skilled in the art that the invention is not limited to these embodiments and that various changes and modifications may be made without departing from the spirit of the invention or the scope of the appended claims.

This application is a continuation-in-part of my previous application Serial Number 582,087, filed May 2, 1956, now abandoned.

I claim:

1. The process of producing terephthalic acid, which consists essentially of heating an alkali metal salt of a benzene polycarboxylic acid with at least three carboxyl groups in the molecule to a temperature between 300 C. and the decomposition temperature of said starting material and the reaction products in a substantially anhydrous, substantially oxygen-free atmosphere of an inert gas converting the alkali metal salts of terephthalic acid formed thereby into the corresponding free acid, and separating said free acid from the reaction mass.

2. The process according to claim 1, wherein the inert gas is carbon dioxide.

3. The process according to claim 1, wherein the inert gas is nitrogen.

4. The process according to claim 1, wherein the inert gas is selected from the group consisting of carbon dioxide, nitrogen, methane, benzene and carbon monoxide.

5. The process of producing terephthalic acid, which consists essentially of heating an alkali metal salt of a benzene polycarboxylic acid with at least three carboxyl groups in the molecule to a temperature above 300 C. but below the decomposition temperature of said starting material and the reaction products in a substantially anhydrous, substantially oxygen-free atmosphere of an inert gas, at elevated pressures, converting the alkali metal salts of terephthalic acid formed thereby into the corresponding free acid, and separating said free acid from the reaction mass.

6. The process of producing terephthalic acid, which consists essentially of heating an alkali metal salt of a 'lected from the group consisting of zinc, cadmium, mercury, iron, lead, cerium, manganese, converting the alkali metal salts of terephthalic acid formed thereby into the corresponding free acid, and separating said free acid from the reaction mass.

7. The method of producing terephthalic acid from mixed aromatic polycarboxylic acids having at least three carboxyl groups per aromatic ring, which consists essentially of converting said aromatic polycarboxy-lic acids into the corresponding alkali metal salts thereof, heating said salts to above 300 C. and below the temperature at which substantial decomposition takes place, in a substantially dry inert atmosphere, to convert said salts into alkali metal salts of terephthalic acid and separating the terephthalic acid from the remainder of the reaction mixture by dissolving said mixture in water and acidifying said mixture to precipitate terephthalic acid therefrom.

8. The process according to claim 1 wherein the benzene polycarboxylic acid is pyromellitic acid.

9. The process according to claim 1 wherein the benzene polycarboxylic acid is mellitic acid.

10. The process according to claim 1 wherein the benzene polycarboxylic acid is hemimellitic acid.

11. The process according to claim 1 wherein the benzene polycarboxylic acid is trimellitic acid.

12. The process according to claim 1 wherein the benzene polycarboxylic acid is trimesitinic acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,885,834 Jaeger Nov. 1, 1932 2,020,505 Jaeger Nov. 12, 1935 2,734,914 McKinnis Feb. 14, 1956 2,863,913 Raecke et al Dec. 9, 1958 

1. THE PROCESS OF PRODUCING TEREPHTHALIC ACID, WHICH CONSISTS ESSENTIALLY OF HEATING AN ALKALI METAL SALT OF A BENZENE POLYCARBOXYLIC ACID WITH AT LEAST THREE CARBOXYL GROUPS IN THE MOLECULE TO A TEMPERATURE BETWEEN 300*C. AND THE DECOMPOSITION TEMPERATURE OF SAID STARTING MATERIAL AND THE REACTION PRODUCTS IN A SUBSTANTIALLY ANHYDROUS, SUBSTANTIALLY OXYGEN-FREE ATMOSPHERE OF AN INERT GAS CONVERTING THE ALKALI METAL SALTS OF TETEPHTHALIC ACID FORMED THEREBY INTO THE CORRESPONDING FREE ACID, AND SEPARATING SAID FREE ACID FROM THE REACTION MASS. 